1. Field of the Invention
The invention relates to an amplitude limiter. More particularly, the present invention relates to an amplitude limiter which reflects a high-frequency signal that exceeds a predetermined threshold.
2. Description Of The Related Art
In many high-frequency applications, particularly in the radar art or in satellite radio, high power high-frequency (HF) transmitters, on the one hand, and highly sensitive high-frequency (HF) receivers, on the other hand, are operated in immediate spatial vicinity. Often HF transmitters and HF receivers are operated in time division multiplex over a common transmit/receive antenna. This is the case, for example, in active, phased array antennas in which, for example, up to several thousand individual antennas (transmit (T) and receive (R) antennas) are combined into an antenna array. With such an antenna array it is possible to pivot the antenna lobe by providing adjustable phase shifters at the individual antennas for the HF signals to be transmitted or received. In such arrangements, mentioned as examples, so-called transmitter (T), receiver (R) or transceiver (T/R) modules perform a very important function. It is often advisable to construct such modules as a unit of the smallest possible three-dimensional size, for example, in the case of a phased array antenna for the nose of an aircraft.
An exemplary configuration with respect to HF for such a T/R module is shown in FIG. 1. This module is essentially composed of:
a receiving path; PA1 a transmission path; PA1 a common control path; and PA1 a common radiator path.
The common radiator path is composed of a radiator ST, a filter FI and a transmit/receive switch, e.g., a circulator ZI.
The receiving path includes an amplitude limiter LI for the HF signal to be received and a subsequently connected highly sensitive low-noise amplifier LNA.
The transmission path includes an amplifier or a series connection of a plurality of amplifiers for the HF signal to be transmitted, e.g., namely a driver amplifier DA, a power amplifier PA and a high power amplifier HPA.
The common control path is composed of a series connection of a phase shifter PS, an HF amplifier AMP and an HF attenuator ATT.
An HF signal Tx to be transmitted travels over a first switch SPDT1, the control path, a second switch SPDT2, the transmission path and the radiator path to the radiator ST, e.g., a horn radiator.
An HF signal received by radiator ST travels over the radiator path, the receiving path, the first switch SPDT1, the control path and the second switch SPDT2 to an output where it appears as an amplified received signal Rx and can be processed further.
Transmitter (S) or receiver (R) modules can be produced by omitting the receiving or transmission path, respectively.
It is evident that in such arrangements the low-noise amplifier LNA present in the receiving path must be specially protected against input signals whose amplitudes are too high and which would lead to destruction or an overshooting condition of amplifier LNA. Such unduly high amplitudes may occur in the receiving path, for example, if the circulator ZI is defective and couples the amplified transmission signal directly into the receiving path. Moreover, the radiator ST may receive an HF signal having an unduly high amplitude, for example, due to a lightning strike or an HF reflector disposed immediately in front of the radiator. It is therefore obvious to protect the amplifier LNA by placing an amplitude limiter LI at its input. The amplitude limiter must have the lowest possible (insertion) attenuation so that the received signal is attenuated as little as possible and, in the receiving mode, the noise figure of the T/R module remains as low as possible.
For that reason, reflecting amplitude limiters are employed. If a maximum permissible HF threshold level is exceeded, such amplitude limiters produce an HF short circuit. This causes the excessive HF power to be conducted in an unwanted manner over the low-resistance path of the circulator ZI to the output end of the transmission path and may damage or even destroy the circulator.